1. Field of the Invention
The present invention relates to a disk drive apparatus for driving a disk mounted thereon to allow signals recorded in the disk to be reproduced.
2. Description of the Related Art
In systems having rotary components, such as portable MD players, noise produced by a spindle motor incorporated in the rotary component is becoming a matter of concern. In a system known as a PWM system in which a spindle motor is driven directly by a pulse-width-modulated signal, a problem also is caused by noise that is generated by a quick variation in electric current occurring at the time of switching of the phase of the electric current.
In order to obviate this problem, a technique known as the “soft-switching” driving or control method has been proposed in which the motor current is reduced immediately before and after a phase switch of the spindle motor.
Such a soft-switching driving method has been implemented by using an analog circuit. The analog circuit is composed of transistors included in a motor driver and, therefore, cannot be formed with a high degree of integration, thus failing to contribute to an appreciable reduction in the cost.
The known circuit implementing the soft-switching driving method involves another problem in that changes or modifications of parameters concerning the driving system by using an external device, such as a microcomputer (MCU), cannot easily be effected because of the fact that, as described above, the circuit is formed in the motor driver in accordance with predetermined design specifications. In other words, modification of the motor-driver circuit is necessary to accommodate any variation in the motor characteristics which may occur.
Current portable mini-disk (MD) players employ as means for driving an MD a brush-less, three-phase motor (referred to simply as “motor”, hereinafter) which does not have any hall element for detecting rotational positions. In operation, the speed of rotation of the motor rotor is computed based on the phase difference between counter-electromotive edge signals which are obtained as a result of operation of the motor, and the timing of switching of the phase is determined in accordance with the rotor speed.
The term “counter-electromotive edge” is used in this specification to mean an output from a comparator which compares an intermediate potential as a reference and a counter-electromotive force that is induced in the coil of each phase. This method of motor control will be referred to as “control under first delay mode”, hereinafter.
It is to be pointed out that any inter-phase offset existing in the comparator for generating detection signals corresponding to rotational positions of the motor rotor tends to cause an undesirable effect, particularly when the rotation speed is low. In other words, error which is involved in the detection signal corresponding to the rotational position of the motor rotor is significant as compared with the case where no offset exists in the comparator. Such a detection signal involving an error may undesirably be masked by a mask signal, which is intended to avoid erroneous detection of the edge, thus hampering correct control of the motor.
A motor control method also has been known in which the motor is controlled by computing a rotation speed based on the time interval between counter-electromotive edges corresponding to one period of a single counter-electromotive edge, i.e., the time interval of counter-electromotive edge corresponding to ¼ rotation of the motor rotor. In the following description, this type of motor control method will be referred to as “control under second delay mode”
In the control under second delay mode, the rotation speed is computed based on the time interval between the counter-electromotive edges corresponding to ¼ rotation of the motor rotor. Therefore, when the motor is abruptly accelerated or decelerated, the counter-electromotive edge tends to be deformed, posing a risk that the phase switching cannot be performed at proper timing.